'extern' declaration at local scope if considered an extremely
poor style in C++. Furthermore, it does not interact
well with template instantiations and scope rules; plus
it does not work well when the function actually has an
internal linkage.

A proper way to bring a symbol into local scope is through
a using-declaration:

using ::gt_pch_nx;

I struggled a bit with this. I need to tell the template function that
there exists a free function gt_pch_nx that takes a T&. This is not
something that any header file has at the point of this declaration.

The function gt_pch_nx(T&) is declared and defined later in one of the
gt-*.h files. This is another problem with the way that gengtype works
today.

The using-declaration that you propose does not seem to give me what I
want, though. How does it know that the function takes a T&?